(2019-12-14, 19:20)Roland Melkert Wrote: The spec is a bit vague, but during LDCad's development I discovered it doesn't really matter where p3 is Y-wise.This was also the conclusion I ended up at. Thankfully the test cases from you and others has helped me a lot arriving at this.
Just use p3 to calculate a direction vector perpendicular to the p1 p2 vector.
And then use that vector to project the texture to the cylinder's surface by getting the angle between it and similar directional vector pointing at a vertex' position.
The issue with the specification regarding p3 is that it is not vague, but says that it corresponds to the bottom center of the texture, which is incorrect. It would be better if the specification mirrored what you just wrote.